Variable current transformer system



y 1, 1950 u. LAMM :rm. 2,514,452

VARIABLE CURRENT TRANSFORMER SYSTEM Filed Sept. 3, 1948 80 .mV MOW VPatented July 11, 1950 VARIABLE CURRENT TRANSFORMER SYSTEM Uno Lamm andHarry Forssell, Ludvika, Sweden,

and Johannes Hansen, Milwaukee, Wis., assignors to Allmanna SvenskaElektriska Aktie bolaget, Vasteras, Sweden, a corporation of SwedenApplication September 3, 1948, Serial No. 47,564 In Sweden June 22, 1942Section 1, Public Law 690, August 8, 1946 Patent expires June 22, 1962Claims. 1

This application is a continuation in part of application S. N. 493,210,filed July 2, 1943, now abandoned.

Transformers having a high and preferably adjustable leakage have, as iswell-known, obtained a vast use among others for welding purposes. Theyhave hitherto most frequently been so constructed that the primary andsecondary windings have been mounted on the different outer legs of athree-legged core, the middle leg of which has contained adjustable airgaps. It has, however, been found that this way affords no possibilityof adjusting the secondary shortcircuit current between wide limits andstill keeping the secondary voltage substantially constant, as isgenerally desirable. According to the present invention, the secondarywinding of the transformer is therefore divided into two or more parts,the leakage fluxes of which are substantially independent of oneanother, and which winding parts can be connected individually orseverally in parallel to the load for a coarse adjustment of thesecondary short-circuit current. A fine adjustment of the said currentis at the same time obtained by variable impedance means included in thecurrent path of one of said winding parts.

We are aware that it has been proposed to regulate the no load voltageof welding transformers by connecting a larger or smaller number ofsecondary turns in series, and also to use certain winding parts on thetransformer core, which have a large leakage with respect to the primarywinding, as series inductances for regulating the current. much widerrange of regulation of the short-circuit current without affecting theno-load voltage by connecting in parallel different numbers of windingparts having a large leakage which determines the short-circuit currentof each part individually, independent of the current in the otherparts.

Two forms of the invention are illustrated in section in Figs. 1 and 2of the accompanying drawing, while Fig. 3 shows a third form.

In Fig. 1, all the parts of the transformer are stationary, while thefine adjustment is efiected by means of an adjustable external seriesinductance. l is the rectangular transformer core and 2 the primarywinding, which comprises two coils on each of two opposite sides of thecore. The secondary winding is also divided into several coils, two ofwhich, designated by 3 and 4, are mounted on the two remaining rectanglesides of the core, thus have a large leakage with re- The presentinvention provides for a spect to the primary winding, while two pairsof secondary coils 5, 6 lie closely adjacent to the primary winding,thus with a small mutual leakage. Between the coil 3 and the load l0, anexternal, constant reactance 1 may be connected, and between the pair ofcoils 6 and the load there may be connnected a reactance 8, adjustablefor instance by means of a superposed direct current for a fineadjustment of the secondary short-circuit current.

The diiierent steps of the coarse adjustment are obtained by closing oneor more of the switches 9. The first step, corresponding to the smallestsecondary current, is represented by secondary coil 3 alone, connectedin series with the external reactance l. The coil 3 then embraces onlysuch a part of the primary flux which is necessary for overcoming thereactance 1, while the rest closes through the air at the bottom. (Thevoltage drop of the load is here neglected, as in welding transformersthe pure short-circuit case must always be taken into account.) To thesecondary current thus obtained, the finely adjustable current is addedwhich flows from the winding 6 through the adjustable reactance 8.

The next step of the coarse adjustment is obtained by bridging thereactance 1 so as to make the coil feed the load directly. The primaryflux is then entirely closed through the air at the bottom.

For the next step, the windings 5 and 4 are connected in series to theload. The winding 5, which has a small leakage with respect to theprimary winding, always generates a voltage, which in the case of fullshort-circuit must be counteracted by the winding 4. The lattertherefore must be traversed by a flux opposite to that of the primarycurrent, corresponding to a higher secondary current than that which canbe produced by the winding 3 alone.

Further coarse adjusting steps can be obtained by connecting the winding4 in parallel to the winding 3, with or without the series reactance I.For each coarse adjusting step, a fine adjusting can be obtained in themanner already described by the reactance 8.

In Fig. 2, all the secondary windings are mounted on the same sides ofthe substantially rectangular transformer core H as the primary windingsl2, but some parts I3, [4 of the secondary windings are mounted at acomparatively large distance from the primary windings, and between theparts I3 and M the core has a projection 2| for increasing the leakage.In the space between one secondary winding I4 and the primary winding,there is a wedge-shaped core portion 22 which is supposed to beadjustable perpendicularly to the plane of the drawing for a fineadjustment of the leakage path around the winding l4. In the mostextracted position of the wedge-shaped core portion 22, the leakagedistance between the corresponding rectangle side and the projection 2|is assumed to be somewhat larger than between the latter and theopposite rectangle side. Two other portions l5, N5 of the secondarywinding are mounted nearer the primary winding so as to have a smallerleakage reactance. All the portions of the secondary winding can besuccessively connected in parallel by switches l3a-I6a to feed the load20.

If the secondary winding I4 is connected in circuit alone by switch Maand the airgap in the leakage path between the said winding and theprimary winding is gradually increased from zero to its maximum value byextracting the wedge 22, the short-circuit current rises continuouslyfrom nearly zero to a value which may be designated by 1. If also thewinding I3 is connected to the load by switch l3a, the current may, byrepeating the same fine-adjustment operation, be gradually raised from 1to 2. The windings l5 and I6, which have a smaller leakage with respectto the primary winding, may each represent the current 2, and thus thewhole adjustment range will be continuous from to 6, the last one whenall the secondary windings are connected in parallel respectively, byswitches [a and Ilia, and the wedge 22 entirely extracted.

The transformer according to the present invention may cooperate with arectifier, and especially in this case it may be advisable to make itthree-phase. In a three-phase transformer it is generally, with respectto the symmetry, most advisable to mount all the windings on the legs ofthe three-legged core, thus essentially in analogy with Fig. 2, but toefiect the fine adjustment by an adjustable inductance, for instance bya transductor (an inductance adjustable by superposed direct current) inanalogy with Fig. 1. In a threephase connection, there is a furtherpossibility of adjusting the leakage besides those possible in asingle-phase connection, namely, a series-connection of two secondarywinding phases, which increases the reactance without increasing theno-load voltage. Such a connection is indicated in Fig. 3, in which 32are the three primary windings of a three-phase transformer having athreelegged iron core 31. The connections of said primary windings tothe feeding three-phase line are not shown, in order to simplify thediagram, but may be in any usual manner, as in star or delta. Each legof the core carries three secondary windings. One of these, designatedby 38, is

.situated close to the primary winding, corresponding to the winding 6in Fig. 1, and is connected in series with a variable reactor 39,represented only in the conventional manner with an arrow to illustratethe variability. Another secondary winding 33 is situated at the top ofthe core at a comparatively large distance from the primary winding, anda third secondary winding 34 is situated at the bottom of the core at astill larger distance from the primary winding. The three secondarywindings should have equal numbers of winding turns but may havedifferent sectional areas of the conductors, corresponding to thedillerent maximum or short-circuit currents to be carried thereby.

The secondary windings 33 are connected to- At the other end, they areconnected to switches 4|. The windings 34 are connected at oneendcorresponding in voltage respect to the flrstnamed end of windings33-each to a doublethrow switch 42, by which they may be connectedeither to a neutral point 44 or to the free (lower) ends of windings 33of another phase. The lower ends of windings 34 are connected toswitches 43, by which they may be connected separately or in parallel tothe windings 33 to a three-phase two-way rectifier 35, between thedirect current terminals of which the load 38 is connected.

When all the switches 4|, 42 are open, the load is fed only by thewindings 38 through the-variable reactors 39 and the rectifier 35. (Insome cases, depending on the nature of the reactors, it may be advisableto employ a separate rectifier for the windings 38, connected inparallel to the rectiher 35 on the direct current side.) The currentfrom the windings 38 may be regulated from zero up to a predeterminedvalue.

The next step in the regulation involves the closing of switches 43while the two-way switches 42 are in their right position. The load willthen be fed by the windings 33 and'34 in series, while the phase anglebetween the voltages of said windings is such that the vector sum oftheir voltages is only equal to each one of the voltage components. Onthe other hand, the total reactance is equal to the sum of thereactances of both windings 33 and 34, and therefore the current israther low. In parallel to the circuits now described, the load is fedfrom the windings 38 through the reactors 39, which will eiiect a fineregulation in the manner already described.

For the next step of regulation, the two-way switches 42 are thrown overto their left position. The windings 34 are then connected to theneutral point and feed alone the load in parallel with the windings 38adapted for the fine regulation. For the following step, the switches 43are opened and the switches 4! closed. Then the windings 33, which aresituated at a smaller distance from the primary windings than are thewindings 34 and therefore have a smaller leakage reactance, feed theload in parallel to the windings 38.

For the final step, giving the highest current value, the switches 4|and 43 are both closed while the switches 42 are still in their leftposition. Then all the secondary windings feed the load in parallel.

We claim as our invention:

1. A transformer having a core, a primary winding on said core, asecondary winding having a plurality of parts on said core, at least twoof said parts having, with respect to the primary winding and to oneanother, relatively large leakage fiuxes, and at least one part having,with respect to the primary winding, a relatively small leakage flux,means forming a plurality of current paths each including at least oneof said parts, means to vary the impedance in one of said paths, andswitch means for selectively connecting various numbers of said currentpaths in parallel to the work, every part of the primary winding havinga relatively small leakage fiux with respect to a part of the secondarywinding.

2. A-transformer according to claim 1 in which the majority of thesecondary winding parts lie on the same core. parts as the primarywinding but at different distances therefrom, and in which the impedancevarying means comprise a variable gether at one end to form a neutralpoint 40. i du t r a by a movable core Portion "tel-posed betweencertain of said secondary winding parts.

3. A transformer having a core, a primary winding on said core, asecondary winding having a plurality of parts on said core, at least twoof said parts having, with respect to the primary winding and to oneanother, relatively large leakage fluxes, and at least one part being sodisposed with respect to the primary'winding as to give a relativelysmall mutual leakage flux, means forming a plurality of current pathseach including at least one of said parts, variable inductance meansconnected in the current path including the secondary winding parthaving the smallest leakage flux with respect to the primary winding,and switch means for selectively connecting various numbers of saidcurrent paths in parallel to the work.

4. A transformer according to claim 3 comprising a rectangulartransformer core, the primary winding being mounted on two oppositesides of said core, and the parts of the secondary winding havingrelatively large leakage fluxes with respect to the primary windingbeing mounted on the two remaining sides of said core.

5. A transformer according to claim 3 comprising a threephase iron core,threephase primary and secondary windings thereon, and means forconnecting in series certain secondary windings diflering 120 in phase.

UNO LAMM. HARRY FORSSELL. JOHANNES HANSEN.

REFERENCES CITED The following references are of record in the Name DateOwen Feb. 14, 1928 McCurtain Nov. 14, 1939 Mills May 14, 1940 Gray May1, 1945 Number

